1. Field of the Invention
The present invention relates generally to differential amplifiers of the type used to amplify a differential input voltage and provide an output voltage for driving a load, and more particularly, to an apparatus and method for canceling any offsets inherent in such differential amplifier.
2. Description of the Relevant Art
Differential amplifiers are widely used within the electronics industry for receiving a differential input signal and producing an amplified output voltage derived therefrom. Often, such differential amplifiers include a first, or positive, input terminal and a second, or negative, input terminal, along with a third output terminal. The differential input voltage is presented across the first and second input terminals, and the output voltage is a function of the difference between the absolute voltages presented at the positive and negative input terminals. Typically, a feedback path is provided from the output terminal back to the negative, or inverting, input terminal to establish the amount of amplification provided by the differential amplifier and to increase the stability thereof
A perfect differential amplifier would provide a null, or mid-range, zeroed output voltage when the magnitude of the input differential voltage is zero. However, due to transistor mismatches, current source mismatches, current mirror errors, and other imperfections, there is usually an input voltage offset inherently present in most differential amplifiers when used in a feedback configuration. This offset error means that the differential input voltage signal must be non-zero (either slightly positive or slightly negative) in order for the output voltage to be brought to the null voltage. This offset error must be compensated if the output voltage is to maintain an accurate linear relationship with the differential input voltage signal.
In the past, the most common method of compensating for the aforementioned offset error is to determine the differential input voltage offset error, and to add or subtract such voltage offset error from one of the differential input terminals. For example, U.S. Pat. No. 4,306,196 to Dwarakanath et al. discloses such circuitry wherein a capacitor is pre-charged with the offset voltage and is disposed in series with the input signal path. One of the disadvantages of such an approach is the need to insert a capacitor within one of the input signal paths, thereby relying upon AC coupling of either the input signal or the feedback signal to the input terminal of the differential amplifier.
Some cancellation circuits require dual or multiple amplifier stages; obviously, such offset compensation circuits do not lend themselves to use within a single stage amplifier. Other cancellation circuits require the feedback loop of the differential amplifier to be closed when detecting the offset error; in such instances, significant stabilization time may be required during the offset detection mode while the feedback loop stabilizes.
Some known offset correction circuits require a virtual ground circuit in order to accomplish cancellation, i.e., one of the input terminals to the differential amplifier must be connected to a reference voltage. However, such circuits do not allow the common mode input to the amplifier to change after the offset error cancellation has been performed.
Offset cancellation circuits are known wherein the feedback loop is broken when deriving the offset correction voltage; U.S. Pat. No. 3,694,760 to Loessi is an example of such a circuit. Other cancellation circuits are known wherein offset correction is achieved by generating offset correcting currents, rather than offset correcting voltages; for example, in U.S. Pat. No. 3,988,689 to Ochi et al., differential drain currents are generated and directed back to the positive and negative input terminals of the differential amplifier to achieve offset error compensation. However, none of such offset cancellation circuits are believed to provide the advantages obtained by the present invention.
Accordingly, it is an object of the present invention to provide a simple and inexpensive differential amplifier circuit which effectively cancels offset error within the differential amplifier.
It is another object of the present invention to provide such a differential amplifier circuit wherein both the differential input signal and the feedback path signal may be directly coupled to the input terminals of the differential amplifier without the use of any AC coupling.
It is still another object of the present invention to provide such a differential amplifier circuit wherein the feedback loop settling time of the differential amplifier is not a factor when detecting the offset error.
A further object of the present invention is to provide such a differential amplifier wherein the common mode level of the differential input signal can change after the offset error has been detected without requiring any resetting of the cancellation offset circuitry.
A still further object of the present invention is to provide such a differential amplifier circuit which can be implemented with a relatively small number of transistors.
Yet another object of the present invention is to provide such a differential amplifier circuit that can achieve offset cancellation with a single-stage differential amplifier.
Still another object of the present invention is to provide a method for compensating the offset error of a differential amplifier which can be implemented without unduly complicating the differential amplifier or degrading its performance.
These and other objects of the invention will become more apparent to those skilled in the art as the description of the present invention proceeds.